1. Field of the Invention
Embodiments of the present invention relates to a liquid crystal display (LCD) device, and more particularly to an apparatus and method for driving an LCD device. Embodiments of the invention are suitable for a wide scope of applications. In particular, embodiments of the invention are suitable for improving the picture quality of an LCD device in accordance with the driving thereof.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices have been developed as alternatives to cathode ray tube (CRT) devices because they are thinner and lighter than their CRT counterparts. An LCD device includes two substrates, and a layer of a liquid crystal material between the two substrates. The liquid crystal material has anisotropic dielectric properties. Accordingly, the LCD device can display a desired image by controlling a light transmittance of the liquid crystal material in accordance with the level of an electric field applied to the liquid crystal material.
The LCD device further includes a plurality of gate lines in one direction on a substrate, and a plurality of data lines insulated from and crossing the gate lines. Crossings of the gate and data lines define pixel regions. Switching devices, such as thin film transistors (TFT), are formed adjacent to the crossings of the gate and data lines.
FIG. 1 shows a circuit diagram of a pixel region of an LCD device according to the related art. Referring to FIG. 1, a switching part, such as a thin-film transistor (TFT), is connected with the n-th gate line (GLn) and the m-th data line (DLm) in the corresponding pixel region and a pixel electrode (P) is connected with the TFT. For example, a gate electrode (g) of the TFT is connected with the n-th gate line (GLn), a source electrode (s) thereof is connected with the m-th data line (DLm), and a drain electrode (d) thereof is connected with the pixel electrode.
The pixel electrode (P) faces a common electrode (C). A liquid crystal material between the pixel electrode (P) and the common electrode (C) forms an equivalent liquid crystal capacitor (Clc) between the pixel electrode P and the common electrode C. Also, a storage capacitor (Cst) is formed in an overlapped portion between the pixel electrode and the (n−1)-th gate line (GLn-1). Also, a parasitic capacitance (Cgd) is formed between the gate electrode (g) and the drain electrode (D) due to misalignment. A common voltage Vcom is applied to the common electrode (C).
The operation of the LCD device including the pixel region will be explained as follows. First, the TFT is turned-on by applying a gate pulse to the gate electrode connected with the corresponding gate line (GLn). Then, a data voltage is applied to the source electrode (s) and is applied to the drain electrode (d). The data voltage is applied to the liquid crystal capacitor (Clc) and the storage capacitor (Cst) through the pixel electrode (P). Accordingly, an electric field is formed by a difference of electric potential between the pixel electrode (P) and the common electrode (C).
If an electric field of the same direction is continuously applied to the liquid crystal material, the liquid crystal material may deteriorate. To prevent the deterioration of the liquid crystal material, the polarity of data voltage is inverted with respect to the common voltage (Vcom).
FIG. 2 shows a circuit diagram of an apparatus for generating a common voltage according to the related art. Referring to FIG. 2, the related art apparatus for generating a common voltage includes a first resistor (R1), a variable resistor (VR), and a second resistor (R2) connected in series between a power voltage terminal (VDD) and ground (VSS). In addition, an output terminal (No) is provided between the variable resistor (VR) and the second resistor (R2).
The first and second resistors (R1, R2) have fixed resistance values, and the variable resistor (VR) has a variable resistance value adjustable by a user. The voltage divided by the resistors (R1, VR, R2) is outputted at the output terminal (No) as a common voltage (Vcom) having a DC level. The common voltage (Vcom) is controlled in accordance with the resistance value of the variable resistor (VR).
The related art apparatus for generating a common voltage may cause a flicker phenomenon in an LCD device. The displayed image on the LCD device flickers whenever a variation in the common voltage (Vcom) causes the data voltage to change state. To decrease the flicker phenomenon, the user has to manually adjust the resistance value of the variable resistor (VR) to achieve an optimum value for the common voltage (Vcom) to be transmitted to the LCD panel. However, manual adjustment is not reliable for providing the optimal common voltage (Vcom) to the LCD panel.
If the common voltage (Vcom) changes, the common voltage (Vcom) is no longer the central value of the data voltage. Thus, the voltage value charged in the pixel electrode changes frame by frame, thereby causing the flicker phenomenon. The flicker phenomenon becomes more and more serious as the LCD panel increases in size.